Fluorinated nanodiamond as a precursor for solid substrate surface coating using wet chemistry

ABSTRACT

The present invention is directed to nanodiamond (ND) surface coatings and methods of making same. Such coatings are formed by a covalent linkage of ND crystals to a particular surface via linker species. The methods described herein overcome many of the limitations of the prior art in that they can be performed with standard wet chemistry (i.e., solution-based) methods, thereby permitting low temperature processing. Additionally, such coatings can potentially be applied on a large scale and for coating large areas of a variety of different substrates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application for patent claims priority to U.S. Provisional PatentApplication Ser. No. 60/627,722, filed Nov. 12, 2004.

The present invention was made with support from the Texas HigherEducation Coordinating Board's Advanced Technology Program and theRobert A. Welch Foundation.

FIELD OF THE INVENTION

This invention relates generally to diamond (surface) coatings, andspecifically to methods of applying such coatings to solid substratesurfaces using wet chemistry.

BACKGROUND

Diamond materials have attracted tremendous attention for centuries dueto their wide range of extreme properties. The applications of diamondare further extended when diamond particles are used to coat substratesurfaces. Currently, the most common methods for the fabrication ofdiamond thin films and coatings use chemical vapor deposition (CVD),involving a gas-phase chemical reaction occurring above a solid surfacewhich becomes coated as a result of diamond deposition (H. O. Pierson,Handbook of Carbon, Graphite, Diamond and Fullerenes. Properties,Processing and Applications. Noyes Publ., Park Ridge, N.J., USA, 1993).While various CVD methods differ in their details, they all share commonfeatures such as sophisticated and costly equipment (e.g., vacuumchamber, reactor, furnace, and heater or plasma generator, gas flowmeters/controllers, etc.), extreme reaction conditions (e.g.,temperature in the range of 1000-1400K), and precise control of gasflow. This group of methods also requires the substrate to be resistantto high temperature and show modest reactivity towards carbon.

Therefore, as a result of the inherent difficulties with theabove-described CVD methods of generating diamond coatings, effortsdirected toward the development of alternative, facile and low-costmethods for coating substrate surfaces with diamond are worth pursuing.

BRIEF DESCRIPTION OF THE INVENTION

In some embodiments, the present invention is directed to nanodiamond(ND) surface coatings and methods of making same. Such coatings areformed by a covalent linkage of ND crystals (crystallites) to aparticular surface via linker species. The methods described hereinovercome many of the limitations of the prior art in that they can beperformed with standard wet chemistry (i.e., solution-based) methods,thereby permitting low-temperature processing. Additionally, suchcoatings can potentially be applied on a large scale and for coatinglarge areas of a variety of different substrates.

In some embodiments, the present invention is directed to methodscomprising the steps of: (a) providing a quantity of fluorinatednanodiamond (i.e., fluoronanodiamond, F-ND) and a functionalized surfacecomprising functional moieties capable of reacting with the fluorinatednanodiamond; and (b) reacting the fluorinated nanodiamond with thefunctionalized surface to form a nanodiamond coating on the surface,said nanodiamond coating comprising nanodiamond crystals covalentlylinked to the surface. In some such embodiments, the functionalizedsurface is selected from the group consisting of a functionalizedceramic surface, a functionalized glass surface, a functionalizedpolymer surface, a functionalized semiconductor surface, afunctionalized metal surface, and combinations thereof. In some suchembodiments, the surface is functionalized with3-aminopropyltriethoxysilane (APTES), and the reacting is carried out ina liquid. In some embodiments, there is a further step of washing toremove any unbound nanodiamond from the surface.

In some embodiments, the present invention is directed to methodscomprising the steps of: (a) providing a quantity of fluorinatednanodiamond; (b) reacting the fluorinated nanodiamond with a linkerspecies to give a nanodiamond-linker complex; and (c) reacting thenanodiamond-linker complex with a surface to form a nanodiamond coatingon the surface, said nanodiamond coating comprising nanodiamond crystalscovalently linked to the surface. In some such embodiments, the surfaceis selected from the group consisting of a ceramic surface, a glasssurface, a polymer surface, a semiconductor surface, a metal surface,and combinations thereof. In some such embodiments, the surfacecomprises functional moieties operable for covalent bonding with thenanodiamond-linker complex. In such embodiments, the reacting steps aretypically carried out in a liquid.

In some embodiments, the present invention is directed to a nanodiamondcoated surface comprising: (a) a surface; and (b) nanodiamondcrystallites, wherein the nanodiamond crystallites are covalently boundto the surface via independent linker species. In some such embodiments,the surface is selected from the group consisting of a ceramic surface,a glass surface, a polymer surface, a functionalized semiconductorsurface, a metal surface, and combinations thereof. In some suchembodiments, the independent linker species is provided by APTES. Insome such embodiments, the average crystallite size in the range of fromabout 2 nm to about 20 nm.

The foregoing has outlined rather broadly the features of the presentinvention in order that the detailed description of the invention thatfollows may be better understood. Additional features and advantages ofthe invention will be described hereinafter which form the subject ofthe claims of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 (Scheme 1) depicts the formation of a nanodiamond-linker complex(e.g., APTES-ND), in accordance with some embodiments of the presentinvention;

FIG. 2 (Scheme 2) depicts the functionalization of a glass surface suchthat the functionalized glass surface can operably react withfluorinated nanodiamond to yield a ND-coated glass surface;

FIGS. 3A and 3B are scanning electron microscopy (SEM) images depicting(A) untreated ND, and (B) fluorinated ND (F-ND);

FIG. 4 depicts infrared spectra of F-ND (Trace a), and the product ofthe reaction between F-ND and APTES (Trace b), i.e., a F-ND-linkercomplex;

FIGS. 5A and 5B are SEM images depicting (A) pure glass, and (B) aND-coated glass surface;

FIG. 6 is an atomic force microscopy (AFM) image of the ND-coated glasssurface depicted in FIG. 5B;

FIG. 7 depicts X-ray photoelectron spectra of ND-coated glass andAPTES-glass, wherein only the former comprises fluorine atoms;

FIG. 8 is an AFM image of a APTES-treated silicon surface;

FIG. 9 is an AFM image of a silicon surface after diamond coating;

FIG. 10 is an SEM image of an APTES-treated steel surface; and

FIG. 11 is an SEM image of a steel surface after diamond coating.

DETAILED DESCRIPTION OF THE INVENTION

In some embodiments, the present invention is directed to nanodiamond(ND) surface coatings and methods of making same. Such coatings areformed by a covalent linkage of ND crystals (i.e., crystallites) to aparticular surface via linker species. The methods described hereinovercome many of the limitations of the prior art in that they can beperformed with standard wet chemistry (i.e., solution-based) methods,thereby permitting low-temperature processing. Additionally, suchcoatings can potentially be applied on a large scale and for coatinglarge areas of a variety of different substrates.

In general terms, methods of coating surfaces with ND, in accordancewith some embodiments of the present invention, comprise the steps of 1)providing a quantity of fluorinated nanodiamond (F-ND) and afunctionalized surface comprising functional moieties capable ofreacting with the fluorinated nanodiamond; and 2) reacting the F-ND withthe functionalized surface to form a ND coating on the surface, said NDcoating comprising ND crystals covalently linked to the surface. In someembodiments, a washing step is employed to remove unbound NDcrystallites. Typically, the ND surface coatings of the presentinvention have a thickness that approximates that of the ND crystallitediameter. See Liu et al., Chem. Mater. 2004, 16, 3924-3930, incorporatedby reference herein, for a description of how to prepare F-ND and forexamples of the chemistry F-ND can undergo. See also Liu et al.,“Fluorinated Nanodiamond as a Wet Chemistry Precursor for DiamondCoatings Covalently Bonded to Glass Surface,” J. Am. Chem. Soc., 2005,127, 3712-3713, incorporated herein by reference; and P. Ball, “Stickyprospects for coats of diamond dust,” Nature, 2005, 434, 580(highlighting Applicants' work).

In some embodiments, the functionalized surface comprises amine moietiescapable of reacting with the fluorine moieties on the F-ND. Saidfunctionalized surface can comprise any combination of surface andfunctional moieties suitable for reacting with F-ND either directly, orthrough additional linker species. An exemplary surface is glass (e.g.,silica glass), which can be in the form of sheets or fibers. Glass caneasily be functionalized with amino group terminated functionalities byway of silane-based chemistry. An exemplary way of functionalizing glassis by reacting it with a species such as 3-aminopropyltriethoxysilane(APTES). Covalent bonds can be formed between the APTES molecule and thesubstrate through the hydrolysis of the APTES ethoxy groups, followed bya coupling condensation involving hydroxyl (—OH) groups on the glasssurface. Other suitable surfaces that can be so coated include, but arenot limited to, silicon and steel.

The present invention provides a convenient and cost-effectivealternative to diamond CVD coating methods. Variations of suchabove-described wet chemistry methods can be extended to the ND coatingof other substrates, such as quartz, silicon, and metals; and a varietyof other surface functionality can be used to bond the fluoronanodiamondto the surface. In some alternate embodiments, the fluoronanodiamond canfirst be reacted with a linker species, such as APTES to form a APTES-NDspecies, that can then be reacted with a suitable surface (in the caseof APTES-ND, the species can be coupled with the —OH groups onunfunctionalized glass by first hydrolyzing the ethoxy groups on theAPTES). Applications for such coatings include, but are not limited to,nano-electromechanical systems (NEMS) and micro-electromechanicalsystems (MEMS), field emission devices, sensors, chemically resistantcoatings, tool coatings (e.g., drill bits), lubricating coatings,semiconducting devices, and applications benefiting from diamond'sexceptional optical properties.

The following examples are provided to demonstrate particularembodiments of the present invention. It should be appreciated by thoseof skill in the art that the methods disclosed in the examples whichfollow merely represent exemplary embodiments of the present invention.However, those of skill in the art should, in light of the presentdisclosure, appreciate that many changes can be made in the specificembodiments described and still obtain a like or similar result withoutdeparting from the spirit and scope of the present invention.

EXAMPLE 1

This example serves to illustrate how nanocrystalline diamond can beproduced. As a source of nanocrystalline diamond (ND) Applicants usednanoscale diamond powder that was produced by explosive detonation (V.Y. Dolmatov, Russian Chemical Reviews 2001, 70, 607). This type ofdiamond powder is commercially available and relatively inexpensive,selling for less than ten dollars per gram. The ND powder describedherein had a purity greater than 97% and was purchased fromNanostructured and Amorphous Materials, Inc. The detonation-synthesizednanodiamond powders comprised clusters of tiny diamond nanocrystals(nanocrystallites). The size of the nanocrystal was around 3.5-6.5 nm,and the average size of the clusters was between 1 and 2 μm, as depictedin the scanning electron microscopy (SEM) image of FIG. 3A.

EXAMPLE 2

This example serves to illustrate how nanocrystalline diamond can befluorinated to yield fluorinated nanodiamond (F-ND). This involved thefluorination of nanodiamond via a reported procedure (Liu, Y.; Agrawal,N.; Gu, Z.; Peng, H.; Khabashesku, V. N.; Margrave, J. L. Rice QuantumInstitute 16th Annual Summer Research Colloquium. Aug. 9, 2002, Houston,Tex., Abstr. p. 2; Khabashesku, V. N. “Functionalization of CarbonNanomaterials for Bio-Medical Applications” in Proceedings ofInternational Meeting of German Society of Gerontology and Geriatrics:Extending the Life Span, Sep. 24-26, 2003, Hamburg, Germany; V. N.Khabashesku, Y. Liu, J. L. Margrave, “Functionalization of NanodiamondPowder through Fluorination and Subsequent Derivatization Reactions,”U.S. Patent Publication No. 20050158549; Khabashesku, V. N.“Functionalization of Carbon Nanomaterials for Bio-MedicalApplications,” Book Chapter in Extending the Life Span, K. Sames, S.Sethe, A. Stolzing (Eds.). LIT Verlag, Münster, Hamburg, Berlin, London,c/o New York. 2005, pp. 147-152; Y. Liu, Z. Gu, J. L. Margrave, V. N.Khabashesku, Chem. Mater. 2004, 16, 3924-3930). After fluorination of NDto yield fluoronanodiamond (F-ND), the cluster size of fluoronanodiamondgrains was significantly decreased (to a few tens of nanometers), withtheir surface terminated by fluorine atoms, as shown in the SEM image ofFIG. 3B. Energy dispersive analysis of X-rays (EDAX) data indicated thatthe F/C ratio in the fluoronanodiamond was about 9.3/100.

EXAMPLE 3

This example serves to illustrate some of the chemistry F-ND canundergo, and how such chemistry can be exploited to form coatings of ND.In previous studies (Khabashesku, V. N. “Fuctionalization of CarbonNanomaterials for Bio-Medical Applications” in Proceedings ofInternational Meeting of German Society of Gerontology and Geriatrics:Extending the Life Span, Sep. 24-26, 2003, Hamburg, Germany; V. N.Khabashesku, Y. Liu, J. L. Margrave, “Functionalization of NanodiamondPowder through Fluorination and Subsequent Derivatization Reactions,”U.S. Patent Publication No. 20050158549; Khabashesku, V. N.“Functionalization of Carbon Nanomaterials for Bio-MedicalApplications,” Book Chapter in “Extending the Life Span”, Ed.: K. Sames,S. Sethe, A. Stolzing (Eds.). LIT Verlag, Münster, Hamburg, Berlin,London, c/o New York. 2005, pp. 147-152; Y. Liu, Z. Gu, J. L. Margrave,V. N. Khabashesku, Chem. Mater. 2004, 16, 3924-3930), Applicants havefound that fluoronanodiamond reacts readily with the amino groups inethylenediamine, as well as those in aminoacids. These molecules becomecovalently bonded to the ND particle surface after elimination of HF.Such chemistry serves as a basis for choosing a suitable moleculecontaining an —NH₂ functional group at one end as a linker moiety for NDsurface bonding. 3-aminopropyltriethoxysilane (APTES) was chosen as sucha linker, not only because it has a terminal amino group, but alsobecause it is a commonly used coupling agent for the modification of avariety of substrate surfaces such as glass, quartz and metals (MetalOrganics for Material & Polymer Technology, Ed. B. Arkles, Gelest Inc.,2001). As mentioned above, covalent bonds can be formed between APTESmolecule and the substrate through the hydrolysis of the ethoxy groupsfollowed by a coupling condensation involving the —OH groups on thesubstrate surface.

To confirm the chemical reaction between APTES and fluoronanodiamond, a“flask” experiment was carried out in a solution (i.e., liquid) phase byadding a slight excess of APTES to the fluoronanodiamond suspension inanhydrous 1,2-dichlorobenzene (ODCB). The reaction flask was kept at130° C. for 24 hours under continuous stirring. The resulting powderedproduct was extensively washed on a glass filter and then dried in avacuum oven overnight.

Fourier transform infrared (FTIR) spectral analyses of the resultingpowder provided clear evidence for the occurrence of this reaction,which leads to the formation of a APTES-ND derivative, as depicted inFIG. 1, Scheme 1. In the FTIR spectrum of fluoronanodiamond (F-ND),shown in FIG. 4, Trace a, a single strong peak is due to the C—F bondstretches. After reaction with APTES, several new peaks corresponding tothe features of APTES appeared in the FTIR spectrum, as shown in FIG. 4,Trace b. Other evidence for such reaction came from EDAX analysis, whichindicated that the fluorine content in the sample was greatly reducedafter reaction, from 9.3/100 in fluoronanodiamond to 1.2/100 (F/C) inAPTES-ND.

Referring to FIG. 2, Scheme 2, after generating fluoronanodiamond, thenext step of the coating process was carried out as follows: a smallpiece (1×1 cm) of a glass microscope slide was cleaned in piranha etch(7:3 v/v 98% H₂SO₄/30% H₂O₂) at 90° C. for 1 hour, rinsed with ultrapurewater (Milli-Q system, Millipore), and dried with a stream of filteredN₂. The substrate was then immersed in a fresh 85 mM solution of APTESin ethanol for 30 minutes, after which it was washed with ethanol,gently dried under a N₂ stream, and cured in a vacuum oven at 100° C.for at least 2 hours to allow for a complete coupling of APTES moleculesto the glass surface. Then, the APTES-treated glass substrate wasimmersed in a fluoronanodiamond/ortho-dichlorobenzene (ODCB) suspensioncreated in a small vial. The vial was sealed and placed in an oven at130° C. for 24-40 hours. After the reaction, the sample was rinsed withethanol and then sonicated in ethanol for 30 minutes in order to removeall unbonded particles deposited on the surface. Finally, the substratewas dried in a flow of nitrogen gas for further measurements.

The ND-coated glass substrate was examined by scanning electronmicroscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectronspectroscopy (XPS). FIGS. 5A and 5B shows an SEM comparison of pureglass (A) to the ND-coated glass substrate (B). From the SEM images, onecan clearly see the difference in appearance before and after thecoating, where a great number of spots are seen on the surface ofND-glass sample. The ND particles, with sizes ranging from 10 to 50 nm,almost completely cover the surface. AFM (FIG. 6) surfacecharacterization data are also consistent with the SEM imageobservations. XPS analysis provides further evidence for the presence ofND particles bonded to the glass surface. The dramatic increase of theat. % carbon measured on the surface was due to diamond (Table 1).Additionally, important data is provided by the observation of afluorine peak in the XPS after coating (FIG. 7). Since the fluorineatoms on the very top of each ND particle will not react with the aminogroups on the glass surface due to steric effects, there should be aconsiderable amount of covalently-bound fluorine remaining on theND—after bonding of the ND to the glass surface. Thus, the observedfluorine peak further proves the presence of the ND particles on theglass surface.

TABLE 1 Element at. (%) content from XPS Elements APTES-glass ND-glassSi2p 26.1 21 O1s 63 50.5 C1s 9.8 26 N1s 1.1 0.4 F1s 0 2.1

EXAMPLE 4

This example serves to illustrate how silicon substrates can also becoated by generally following the above-described coating procedure.FIGS. 8 and 9 are AFM images of a silicon surface before and aftercoating, respectively. It is clearly seen that there are particles onsaid surface after reaction. However, the particles are isolated likeislands, which is quite different from the continuous coatings seen oncoated glass substrate. It is believed that the silicon surface is muchmore oxidation resistant than that of glass, which suggests that theremight not be plenty of hydroxyl groups for surface silanation.Therefore, it is likely that fluoronanodiamond particles can only beattached to those areas with enough amino groups.

EXAMPLE 5

This example serves to illustrate how the above-described coatingmethods can also be employed to form nanodiamond coatings on steel. Uponcarrying out such coating processes on steel, differences were seenbefore and after reaction, as verified the SEM images of FIGS. 10 and11, respectively.

All patents and publications referenced herein are hereby incorporatedby reference. It will be understood that certain of the above-describedstructures, functions, and operations of the above-described embodimentsare not necessary to practice the present invention and are included inthe description simply for completeness of an exemplary embodiment orembodiments. In addition, it will be understood that specificstructures, functions, and operations set forth in the above-describedreferenced patents and publications can be practiced in conjunction withthe present invention, but they are not essential to its practice. It istherefore to be understood that the invention may be practiced otherwisethan as specifically described without actually departing from thespirit and scope of the present invention as defined by the appendedclaims.

1. A nanodiamond coated surface comprising: a) a substrate surface; andb) fluorinated nanodiamond crystallites, wherein the fluorinatednanodiamond crystallites are covalently bound to the substrate surfacevia a silane linker species; and wherein the silane linker speciesfurther comprises an amine that is covalently bound to the fluorinatednanodiamond crystallites.
 2. The nanodiamond coated surface of claim 1,wherein the substrate surface is selected from the group consisting of aceramic surface, a glass surface, a polymer surface, a functionalizedsemiconductor surface, a metal surface, and combinations thereof.
 3. Thenanodiamond coated surface of claim 1, wherein the substrate surface isselected from the group consisting of a glass surface, a siliconsurface, a steel surface, and combinations thereof.
 4. The nanodiamondcoated surface of claim 1, wherein the silane linker species comprisesAPTES.
 5. The nanodiamond coated surface of claim 1, wherein an averagesize of the fluorinated nanodiamond crystallites ranges from about 2 nmto about 20 nm.
 6. The nanodiamond coated surface of claim 1, whereinthe substrate surface is coated with the silane linker species; whereineach of the silane-linker species is covalently bound to the substratesurface; and wherein the amine reacts with a quantity of the fluorinatednanodiamond crystallites while the silane linker species is covalentlybound to the substrate surface to form the nanodiamond coated surface.7. The nanodiamond coated surface of claim 1, wherein the fluorinatednanodiamond crystallites are covalently bound to the amine to form ananodiamond-linker complex; and wherein the nanodiamond-linker complexis reacted with the substrate surface to form the nanodiamond coatedsurface; and wherein the nanodiamond-linker complex is covalently boundto the substrate surface.
 8. The nanodiamond coated surface of claim 5,wherein the fluorinated nanodiamond crystallites form a coating on thesubstrate surface; wherein the coating has a thickness of about theaverage size of the fluorinated nanodiamond crystallites.